The storage of hydrogen in the form of metal hydrides has gained a lot of attention in the recent years. The storage of hydrogen in metal hydrides is based on chemisorption, i.e. no molecular hydrogen (H2) is stored but the hydrogen reacts with the metal to form metal hydrides. Storage of hydrogen in the from of metal hydrides has the advantage over storage of hydrogen in for instance liquid or compressed state in that it does not require the use of low temperatures or excessive pressures.
In U.S. Pat. No. 6,106,801 it is disclosed that Ti-doped NaAlH4 compositions can be used to store hydrogen. U.S. Pat. No. 6,106,801 discloses that by doping NaAlH4 with Ti the hydrogen desorption temperature decreases from approximately 200° C. to 140° C. However, Ti-doped NaAlH4 may comprise hydrogen atoms up to a maximum hydrogen storage density per weight of storage material of approximately 5 wt %.
It has been proposed by Chlopek et al. (J. Mater. Chem., 2007, 17, 3496-3503) that a suitable alternative would be the use of magnesium tetrahydroborate, i.e. Mg(BH4)2. This hydride may comprise up to 14.9 wt % of hydrogen, based on the weight of the hydride. However, the onset temperature of hydrogen desorption is high, typically temperatures above 290° C. are required before hydrogen is released from the hydride.
Li et al. (Li et al., Dehydriding and rehydriding processes of well-crystallised Mg(BH4)2 accompanying with formation of intermediate products, Acta Mater (2008) doi10.1016/j.actamat. 2007.11.023) show that well-crystallized Mg(BH4)2 may be dehydrided. In a second step the dehydrided Mg(BH4)2 is rehydrided by subjecting the dehydrided well-crystallized Mg(BH4)2 to hydrogen at a temperature of 543 K and a pressure of 40 MPa for a time period of 48 hours. In a third, dehydriding, step, 6.1 mass % of hydrogen could be obtained from the material, which was rehydrided in the second step. Of the 6.1 mass %, 3.9 mass % was attributed to the formation of MgH2 during rehydriding in the second step. Disadvantage of the process of Li et al. is that this only modest rehydriding takes place under severe pressure and temperature conditions for prolonged times. Furthermore, it is undesired to form MgH2 during rehydriding. MgH2 has a much lower hydrogen storage capacity than the well-crystallized Mg(BH4)2.
There is still a need in the art for a hydrogen storage material that allows a reversible storage of hydrogen at low hydrogen uptake and release temperatures and mild rehydriding conditions.